As far as I can tell by RAW you only get to defend with reaction because the NPC is using a ranged weapon.
QUOTE (pg153 SR4A)
Normally no skill applies to defending against ranged attacks—defending characters simply roll their Reaction (the defaulting modifier does not apply). However, characters may chose to go on full defense (p. 160), thereby adding their Dodge (or Gymnastics) skill to the equation.
QUOTE (pg156 SR4A)
Whenever two or more characters engage each other in hand-to-hand combat or armed combat that does not involve ranged weapons, the following melee combat rules apply.
QUOTE (pg159 SR4A)
Melee combat weapons are any weapons wielded by combatants within a few meters of each other. Some of these weapons can also be thrown, but if the weapon is swung, melee combat rules apply. If you throw a melee weapon (a club, for instance), you resolve that action as a ranged projectile attack (see Projectile Weapons, p. 155, and Ranged Combat, p. 150).
I also find this statement quite Odd(Particularly the bold section):
QUOTE (pg156 SR4A)
Defenders have three choices for defending against unarmed attacks. If they have a melee weapon in hand, they can parry the attack by rolling the appropriate weapon skill + Reaction. If they have Unarmed Combat skill, they can choose to block by rolling Unarmed Combat + Reaction. Or they can simply dodge out of the way using Dodge + Reaction.